1. Field of the Present Invention
The present invention relates generally to the field of gas sensors, and, in particular, to the art of detecting halogenated refrigerants by applying control theory to an improved xe2x80x9cheated electrodexe2x80x9d technology to control the operation of the detector using an advanced sensing device and one or more control loops.
2. Background Art
Gas detectors for sensing the presence of halogenated gases and other gases are well known. FIG. 1 shows prior art gas detector type suitable for this purpose, commonly referred to as a xe2x80x9cheated electrodexe2x80x9d sensor. This sensor utilizes a cathode wire and an anode wire made of platinum, palladium or an alloy thereof. Typically, the cathode is repeatedly coated with a ceramic material such as a mixture of an alkali metal silicate and oxides of aluminum or silicon, with a drying period between each coat, and then inserted into an anode coil formed by several turns of the anode wire. The anode/cathode assembly is then coated further with the same mixture, except for the ends of the anode and the exposed end of the cathode, and dried. After the final drying, the anode/cathode assembly is fired in a kiln and then installed in a housing, with the exposed ends of the anode and cathode connected to anode contacts and a cathode contact, respectively. The final assembly is then energized and biased over many hours by applying a electrical current through the anode coil and a voltage across the anode coil to the cathode wire.
The ceramic forms an electrically resistive layer between the electrodes. When heated by an electrical current passing through a first of the electrodes, an outer layer depleted of ions develops along the electrodes. When this layer is exposed to reactive gases like halogen, ions flow across the depletion zone and the conductivity of the device is increased. Thus, the presence of halogenated gases may be determined by monitoring the current generated through the second electrode, referred to as the bias current, for a sudden increase in magnitude created by introducing the device to such gases. These sensors are commonly utilized by technicians to determine whether a refrigerant leak exists and to pinpoint its source.
Advantageously, heated electrode sensors have low electrical power requirements and good sensitivity, and such sensors exhibit excellent selectivity in that they tend to ignore most chemical vapors which may be present in a typical test environment, as well as water vapor. Unfortunately, prior art heated electrode sensors also suffer a number of drawbacks. First, and most significantly, the bias current is dependent not only upon the presence or absence of halogenated molecules at the electrodes, but by the temperature of the device as well. Thus, sudden changes in temperature are frequently misinterpreted as an indication of the presence of halogenated molecules because their respective effects are the same: each causes an increase in the bias current of the sensor.
U.S. Pat. No. 4,305,724 to Micko (the xe2x80x9c""724 patentxe2x80x9d) discloses a combustible gas detection system including a sensor temperature control system. The detection system includes a sensor element having active and reference sensors for detecting combustible gases, a controlled current source for providing electrical power to the sensor element, a voltage-to-duty cycle converter for providing a square wave control signal of variable duty cycle and a bypass switch for bypassing the active sensor element in response to the control signal. By increasing or decreasing the duty cycle, the amount of electrical energy flowing to the active element is likewise affected and the temperature of the active sensor may correspondingly be either upwardly or downwardly biased. When the presence of combustible gas begins to cause the temperature of the active sensor to increase, the increase is detected by the temperature control system and the duty cycle is adjusted to counteract the increase and maintain the temperature constant.
Unfortunately, the detection system of the ""724 patent suffers from some drawbacks. First, the detection system of the ""724 patent requires the use of a reference sensor. Perhaps more importantly, the temperature control system is used only to equalize the temperature of one sensor with respect to the other sensor. In particular, it includes no means for measuring the absolute temperature of either sensor, or for independently setting the absolute temperature of either sensor to a particular chosen value. This is sufficient in the active sensor type of the ""724 patent because the presence of the gas sought may generally be indicated merely by the heat given off by the oxidation process, as indicated by the temperature of the active sensor compared to that of the reference sensor. This characteristic makes the active sensor of the ""724 patent impervious to fluctuations in absolute temperature due to ambient conditions. However, in heated electrode refrigerant detector systems, the presence of the gas sought is indicated generally by an increase in bias current, which is also affected by the ambient temperature of the sensor. As a result, a heated electrode refrigerant sensor using the temperature control system of the ""724 patent would still be affected by ambient conditions because it is incapable of controlling the absolute temperature of the sensor. In addition, the absolute temperature of the sensor cannot be controlled to prevent damage during warm-up of the system and the like. Thus, a need exists for a temperature control system suitable for use with a heated electrode refrigerant detection system which does not make use of a reference sensor and which may be utilized to control the absolute temperature of the heated electrode.
U.S. Pat. No. 3,912,967 to Longenecker (the xe2x80x9c""967 patentxe2x80x9d) discloses a circuit for providing regulation of the absolute temperature of a heater-anode of a refrigerant gas sensor. A power supply outputs two different DC voltage levels, one of which is connected through a transistor switch to the heater-anode coil of a heated electrode gas sensing element. The circuit monitors the approximate absolute temperature of the heater-anode based on its effective resistance. When the absolute temperature of the heater-anode drops enough below a desired value, a temperature regulation circuit closes the switch, and a greater amount of current is supplied to the heater-anode. When the temperature of the heater-anode reaches the desired value again, the temperature regulation circuit opens the switch and a lesser amount of current is supplied to the heater anode. Thus, as the temperature of the sensing element fluctuates, greater or lesser heating may be applied to the heater-anode by the temperature regulation circuit. Unfortunately, although this circuit provides some control over the absolute temperature of a heated electrode refrigerant sensor, the regulation is relatively crude, effectively permitting control only by turning an auxiliary heat source on and off. At best, the temperature of the sensor is thus roughly held in a general range, with the upper approximate limit being the desired temperature and the lower approximate limit being the temperature at which the transistor of the switch is cool enough to allow the auxiliary power supply to be coupled in. At worst, however, such a crude controller may allow the temperature of the sensor to oscillate wildly and even dangerously under certain conditions. Further, the circuit allows no adjustment to be made to the triggering temperatures at which the auxiliary source is turned on or off. Thus, a need exists for a more sophisticated temperature control system suitable for use with a heated electrode refrigerant detection system which allows the temperature of the sensor to be rigidly maintained at a particular absolute value, rather than within a wide range of temperatures, and wherein that value is adjustable.
Another disadvantage of prior art heated electrode sensors is that their lifespans are frequently limited much more than is necessary. It is well known that the operation and lifespan of heated electrode sensors are limited by the number of alkali ions in the sensor. It has been found that the bias current and the rate of depletion of ions are directly related to each other. Thus, as the sensor is used, the ions are depleted, and when no ions are left at all, the sensor is xe2x80x9cdead.xe2x80x9d Unfortunately, the sensitivity of the sensor is directly related to the bias current, and so the greater the sensitivity of the sensor, the more quickly the sensor is used up. Prior art heated electrode sensors fail to take these characteristics into account and are thus used up more quickly than is necessary. In addition, the exposure of prior art sensors to high concentrations of refrigerant, even for a relatively short period of time, causes a correspondingly high bias current which results in an immediate reduction in sensor sensitivity and a considerable shortening of the sensor""s lifespan. This effect is known in the industry as xe2x80x9cpoisoningxe2x80x9d the sensor, and no good solution to the problem has yet to be proposed. Finally, despite their limited lifespan, prior art refrigerant detectors provide no means of monitoring or checking the sensor to determine its remaining life.
Some solutions to these problems have been proposed. For example, the H10Xpro Refrigerant Leak Detector, available from the Yokogawa Corporation of America of Newnan, Ga., is a refrigerant leak sensor of the heated electrode type. Like other sensors of this type, the Yokogawa sensor becomes less sensitive over time. The Yokogawa sensor allows users to increase the sensitivity of the sensor by increasing the heat which is applied to the electrode. Because the magnitude of the bias current is dependent not only on the voltage potential between the anode and cathode and the amount of refrigerant present, but is also dependent upon the temperature of the electrode, and because the sensitivity of the sensor is related to the magnitude of the bias current, the sensitivity of the sensor may be improved by raising the temperature of the electrode during operation of the sensor. Yokogawa allows this to be done by manually turning a screw a small amount, presumably to adjust the operating voltage of the electrode. Further, there is a great danger that the user may forget to return the sensor temperature to the manufacturer""s setting when he replaces a depleted sensor with a new one, therefore operating the new sensor at a highly elevated temperature and seriously reducing the life of the new sensor. An improved sensor which continually and automatically adjusts the operation of the electrode to provide sufficient sensitivity over an extended lifetime of the sensor is needed.
U.S. Pat. No. 3,739,260 to Schadler (the xe2x80x9c""260 patentxe2x80x9d) discloses a method of operating a halogen detector of the heated electrode type. A current supply unit supplies current through a current setting means to the electrode to heat the anode, thus creating a fundamental ion current flow between the anode and the cathode. The presence of halogenous gas at the electrode causes an increase in the ion current which is amplified and its magnitude indicated by an indicator and/or an alarm. In addition, another amplifier is connected in a feedback loop between the output of the electrode and the current setting means. When the magnitude of the ion current varies by a predetermined amount, the variable gain amplifier supplies a signal to the current setting means to adjust the heating supply current to the anode in a direction to counteract the variation. Unfortunately, the detector of the ""260 patent suffers from some serious drawbacks.
First, because at power-on there is typically a leakage current which flows through the electrode, the feedback loop will operate to adjust the supply current to maintain the ion current at the level of that leakage current. It has been discovered that the leakage current is due to the absorption of moisture while the detector is not in use, and is generally many times larger than the bias current required for normal operation. Therefore, the xe2x80x9cvariable gain amplifierxe2x80x9d described may never provide enough gain at power-on to raise the temperature of the sensor to its desired operating point.
Significant limitations are also placed on the performance of the detector of the ""260 patent by the means by which a refrigerant is detected. More particularly, not only is the ion current being controlled by the feedback loop, but it is also the process variable which is monitored for a condition indicating the presence of halogen molecules. Unfortunately, such an approach mandates the use of inherent high-pass filtering artifacts that reduce a signal level change into a time-varying peak which lasts only a certain period of time, even though refrigerant may still be present at the sensor. Further, the detector of the ""260 patent is designed to compensate only for relatively slow fluctuations of the ion current and no adjustment is made by the feedback loop for spikes in the magnitude of the ion current which disappear before the end of the period of the gain amplifier is reached. The single process variable approach thus permits short term, high-magnitude fluctuations in the ion current which significantly shorten the lifespan of the sensor. Thus, a more sensitive and longerlasting heated electrode leak detector is needed which uses a control loop and a plurality of process variables to more reliably detect the presence of a refrigerant.
Finally, another drawback of prior art sensing devices is the length of time required to assemble and xe2x80x9cburn inxe2x80x9d a anode/cathode assembly. Existing methods require both the anode and the cathode to be coated with the ceramic material before assembly and then further coated thereafter and require considerable periods of time for drying between the various coatings. Further, prior art methods require an assembled anode/cathode assembly to first be fired in order to sinter the ceramic material before biasing and the assembly to create a depletion region. A need exists for a manufacturing method which may be completed in a much shorter period of time than is possible using known methods.
Briefly summarized, the present invention relates to a gas detector having a heated electrode sensing device for sensing the presence of one or more predetermined gas and one or more control loops for controlling the operation of the sensing device. Broadly defined, the gas detector according to one aspect of the present invention is operative in conjunction with a power source and includes: a detection circuit, the detection circuit including a sensing device having first and second electrodes, wherein the first electrode is connected to the power source for heating the first electrode; a temperature controller operatively connectable to the detection circuit for maintaining a temperature of the first electrode at a predetermined magnitude; and a current controller operatively connectable to the detection circuit for maintaining a current in the second electrode at a predetermined magnitude.
In features of this gas detector, the temperature controller is operatively connected to the detection circuit during a first mode of operation, and the current controller is operatively connected to the detection circuit during a second mode of operation; the first mode of operation is a warm-up phase, and the second mode of operation is a normal operation phase; the gas detector has a switch adjustable between at least two positions, wherein in a first switch position the temperature controller is operatively connected to the detection circuit and in a second switch position the current controller is operatively connected to the detection circuit; the position of the switch is determined on the basis of an operating condition of the gas detector; and the sensing device includes a cathode wire, an anode wire at least partly surrounding the cathode wire and having opposing ends, a pair of supply contacts electrically connected to respective ends of the anode wire, a pair of temperature sense contacts electrically connected to respective ends of the anode wire, and a cathode contact electrically connected to an end of the cathode wire.
The present invention also includes a method of controlling the operation of a gas sensing device, the gas sensing device for indicating the presence of a gas of a predetermined type, wherein the method includes the steps of: adjustably heating the gas sensing device; generating a bias current; controlling the temperature of the heated gas sensing device on the basis of at least one operating condition of the sensing device; and controlling the bias current generated by the heated gas sensing device on the basis of at least one operating condition of the sensing device.
In features of this method, the temperature controlling step includes the step of maintaining the temperature of the heated gas sensing device at a predetermined absolute temperature; the method further comprises the step of moving the sensing device into the presence of a gas of a predetermined type, and the bias current controlling step includes the step of maintaining the magnitude of the bias current at a generally constant level during the moving step; generating a signal at least partially representative of the temperature of the sensing device and monitoring the signal for an indication of the presence of at least one predetermined gas; the steps of controlling the temperature of the heated gas sensing device and controlling the bias current generated by the heated gas sensing device occur sequentially; and the transition from one of the controlling steps to the other occurs on the basis of at least one operating condition of the sensing device.
In another aspect of the present invention, a controller for controlling the operation of a gas detector, the gas detector for indicating the presence of a gas of a predetermined type and having a heated gas sensing device generating a bias current, includes: a temperature control loop for controlling the temperature of the heated gas sensing device on the basis of at least one operating condition of the sensing device; and a bias current control loop for controlling the bias current generated by the heated gas sensing device on the basis of at least one operating condition of the sensing device.
In features of this aspect, the temperature control loop is operatively connected to a detection circuit during a first mode of operation, which may be a warm-up phase, and the bias current control loop is operatively connected to the detection circuit during a second mode of operation, which may be a normal operation phase; the controller has a switch adjustable between at least a first switch position in which the temperature control loop is operatively connected to a detection circuit and a second switch position in which the bias current control loop is operatively connected to the detection circuit; the position of the switch is determined on the basis of an operating condition of the gas detector; the sensing device includes a cathode wire, an anode wire at least partly surrounding the cathode wire and having opposing ends, a pair of supply contacts electrically connected to respective ends of the anode wire, a pair of temperature sense contacts electrically connected to respective ends of the anode wire, and a cathode contact electrically connected to an end of the cathode wire, and the temperature control loop is electrically connected to the temperature sense contacts; and an output of the bias current control loop is electrically connected to an input of the temperature control loop.
The present invention also includes a method of controlling a gas detector for sensing the presence of at least one predetermined gas, the gas detector having a heated first electrode and a second electrode, wherein the method includes the steps of: heating the first electrode to a predetermined absolute temperature; upon reaching the predetermined absolute temperature, placing the electrodes in a test location; upon being exposed to one of the predetermined gases, generating an increased current in the second electrode; and maintaining the first electrode at substantially the predetermined absolute temperature while placing the electrodes in the test location and while generating the increased current.
In features of this method, the method further includes the steps of selecting the predetermined absolute temperature and, while the detector is being operated, providing an indication of the predetermined absolute temperature to the gas detector; the step of providing an indication of the predetermined absolute temperature includes the step of predefining the predetermined absolute temperature during manufacturing; the step of providing an indication of the predetermined absolute temperature includes the step of entering the predetermined absolute temperature into the gas detector; the amount of heat applied to the first electrode is dependent on a duty cycle, and the step of maintaining the first electrode at substantially the predetermined absolute temperature includes the step of adjusting the duty cycle; the method further includes the step of monitoring the actual temperature of the first electrode, and the step of maintaining the first electrode at substantially the predetermined absolute temperature includes the steps of reducing the temperature of the first electrode upon determining that the actual temperature exceeds the predetermined absolute temperature and raising the temperature of the first electrode upon determining that the actual temperature is below the predetermined absolute temperature.
The present invention also includes a method of controlling a heated electrode gas detector for sensing the presence of at least one predetermined gas, the gas detector having first and second electrodes, wherein the method includes the steps of: selecting a preferred absolute temperature; providing an indication of the selected preferred absolute temperature to the gas detector; adjustably heating the first electrode; upon being exposed to one of the predetermined gases, generating an increased current in the second electrode; monitoring the temperature of the first electrode while the increased current is being generated; comparing the monitored temperature to the selected preferred absolute temperature; and varying the heating of the first electrode on the basis of the outcome of the comparing step.
In features of this method, the step of providing an indication of the selected preferred absolute temperature includes the step of entering a value corresponding to the selected preferred absolute temperature into the gas detector; the step of providing an indication of the selected preferred absolute temperature includes the step of predefining the selected predetermined absolute temperature to the gas detector during manufacturing; the step of varying the heating of the first electrode includes the steps of reducing the temperature of the first electrode upon determining that the monitored temperature exceeds the selected preferred absolute temperature and raising the temperature of the first electrode upon determining that the monitored temperature is below the selected preferred absolute temperature; at least the monitoring, comparing and varying steps are repeated substantially continuously during operation of the gas detector; the selected preferred absolute temperature is a first preferred absolute temperature, and the method further includes the steps of selecting a second preferred absolute temperature, providing an indication of the second selected preferred absolute temperature to the gas detector, adjustably heating the first electrode, generating an increased current in the second electrode upon being exposed to any of the predetermined gases, monitoring the temperature of the first electrode while the increased current is being generated, comparing the monitored temperature to the second selected preferred absolute temperature, and varying the heating of the first electrode on the basis of the outcome of the comparing step.
The present invention also includes a method for sensing the presence of at least one predetermined gas at a sensing device having first and second electrodes, wherein the method includes the steps of: heating the first electrode; generating, at the second electrode, a bias current; moving the sensing device into the presence of one of the predetermined gases; maintaining the magnitude of the bias current at a generally constant level during the moving step; generating a signal at least partially representative of the temperature of the sensing device; and monitoring the temperature signal for an indication of the presence of at least one predetermined gas.
In features of this method, the presence of a predetermined gas is indicated by a decrease in temperature; the bias current is a first signal, and the temperature signal is a second signal; the first electrode includes at least two ends, and the generating step includes generating the temperature signal at one or more of the ends of the first electrode; the step of generating the bias current includes the step of generating the bias current according to a duty cycle, and the step of maintaining the magnitude of the bias current at a generally constant level includes maintaining the magnitude of the bias current at a generally constant level according to the value of the duty cycle.
The present invention also includes a method for sensing the presence of at least one predetermined gas at a sensing device having first and second electrodes, wherein the method includes the steps of: heating the first electrode; generating, at the second electrode, a bias current; generating a first signal at least partially representative of the magnitude of the bias current, the magnitude of the bias current being a first operating condition; generating a second signal at least partially representative of a second operating condition; maintaining the magnitude of the bias current at a generally constant level on the basis of the first signal; and monitoring the second signal for an indication of the presence of at least one predetermined gas.
In features of this method, the second operating condition is a temperature of the sensing device; the presence of a predetermined gas is indicated by a decrease in temperature; the first electrode includes at least two ends, and the step of generating a second signal includes generating the second signal at one or more of the ends of the first electrode; the method further includes the step of moving the sensing device into the presence of one of the predetermined gases, and the maintaining step includes maintaining the magnitude of the bias current at a generally constant level during the moving step; the step of generating the bias current includes the step of generating the bias current according to a duty cycle, and the step of maintaining the magnitude of the bias current at a generally constant level includes maintaining the magnitude of the bias current at a generally constant level according to the value of the duty cycle.
The present invention also includes a method of estimating the remaining useful life of a heated electrode gas detector for sensing the presence of at least one predetermined gas, the gas detector having first and second electrodes, wherein the method includes the steps of: adjustably heating the first electrode to maintain a current in the second electrode of a predetermined magnitude, the magnitude of the current being at least partly dependent upon the temperature of the first electrode; while heating the first electrode, determining information at least partly representative of the operating temperature of the gas detector; comparing the operating temperature information to information representative of a maximum operating temperature; and determining the remaining useful life of the gas detector on the basis of the comparison.
In features of this method, the information at least partly representative of the operating temperature of the gas detector and the information representative of the maximum operating temperature are both particular values; the determining information step includes sensing the actual operating temperature of the gas detector; the information at least partly representative of the operating temperature of the gas detector and the information representative of the maximum operating temperature are both particular temperature values; the information at least partly representative of the operating temperature of the gas detector is a particular duty cycle value, which corresponds to the operating temperature of the gas detector; the step of comparing the temperatures includes subtracting the operating temperature value from the maximum operating temperature value; the step of determining the remaining useful life includes determining the remaining useful life of the gas detector as a function of the difference between the operating temperature value and the maximum operating temperature value; the method further includes the step of predetermining the maximum operating temperature; the step of predetermining the maximum operating temperature is done empirically; and the maximum operating temperature is a maximum safe operating temperature of the gas detector and/or the maximum operating temperature is a maximum effective operating temperature of the gas detector.
In another aspect of the present invention, a gas detector for sensing the presence of at least one predetermined gas and operative in conjunction with a power source, includes an anode/cathode assembly coated with a ceramic material, the anode/cathode assembly having a cathode wire and an anode wire at least partly surrounding the cathode wire, wherein the anode wire has opposing ends and wherein one of the anode wire ends is electrically connected to the power source; a pair of supply contacts electrically connected to respective ends of the anode wire; a pair of temperature sense contacts electrically connected to respective ends of the anode wire; a cathode contact electrically connected to an end of the cathode wire; and a temperature-sensing circuit electrically connected to at least one of the temperature sense contacts for monitoring the temperature of the anode/cathode assembly.
In features of this aspect, the power source is electrically connected to at least one of the supply contacts; the gas detector has a bias current-sensing circuit electrically connected to the cathode contact; the gas detector has a current source electrically connected to at least one of the supply contacts; and the gas detector has a switch for bypassing the current source.
The present invention also includes a method of making a sensing device for a heated electrode gas detector, the method including the steps of: inserting a cathode wire into an uncoated anode coil to form an electrode assembly; after inserting the cathode wire into the uncoated anode coil, coating the electrode assembly with a ceramic material; and firing the coated electrode assembly.
In features of this method, the inserting step includes inserting an uncoated cathode wire into the uncoated anode coil to form the electrode assembly; the firing step is accomplished by applying a heating current to the anode coil; the method includes the step of biasing the coated electrode assembly by applying a biasing voltage to the electrode assembly; and the steps of firing and biasing are carried out substantially entirely simultaneously.
The present invention also includes a method of making a sensing device for a heated electrode gas detector, the method including the steps of: inserting a cathode wire into an anode coil to form an electrode assembly; coating at least part of the cathode wire and at least part of the anode coil with a ceramic material to form an unfired electrode assembly; and biasing the unfired electrode assembly to form a depletion region.
In features of this method, the biasing step includes biasing the unfired electrode assembly by applying a biasing voltage to the anode coil; the method further includes the step of firing the unfired electrode assembly by applying a heating current to the anode coil; and the steps of firing and biasing are carried out substantially entirely simultaneously; the firing and biasing steps are completed within one hour.
The present invention also includes a method of efficiently preparing a heated electrode refrigerant detector for use, the detector including a sensing device, wherein the method includes the steps of: determining a first temperature, the first temperature being a desired sensing device operating temperature; determining a second temperature, the second temperature being higher than the first temperature; gradually raising the actual temperature of the sensing device from a third temperature until the second temperature is reached, wherein the third temperature is substantially less than the first temperature; and after reaching the second temperature, lowering the actual temperature of the sensing device until the first temperature is reached.
In features of this method, the second temperature is generally equal to the maximum sustainable operating temperature of the sensing device; and the third temperature is the ambient temperature of the sensing device before the sensing device is heated.
The present invention also includes a method of preparing a heated electrode refrigerant detector for use, the detector including a sensing device, wherein the method includes the steps of: maintaining the actual temperature of the sensing device at a first temperature; while maintaining the actual temperature of the sensing device at the first temperature, generating a bias current, the bias current decreasing in magnitude over time; monitoring the bias current; and on the basis of the monitored bias current, reducing the actual temperature of the sensing device to a second temperature which is a desired sensing device operating temperature.
In features of this method, the first temperature is generally equal to the maximum safe operating temperature of the sensing device; the temperature reducing step is executed on the basis of the negative slope of the monitored bias current over time being less than a predetermined value; and the temperature reduction is effected by reducing the magnitude of the bias current to a desired operating level.
The present invention also includes a method of re-polarizing a heated electrode refrigerant detector having a sensing device operable at an operating temperature, the method including the steps of: elevating the temperature of the sensing device above the operating temperature until the sensing device is substantially re-polarized; and decreasing the temperature of the sensing device to the operating temperature.
In features of this method, the method further includes the step of monitoring the magnitude of a bias current generated by the sensing device, and the initiation of the step of decreasing the temperature of the sensing device is dependent at least partly upon the magnitude of the bias current; and the method further includes the step of monitoring the amount of time for which the temperature of the sensing device is elevated above operating temperature, and the initiation of the step of decreasing the temperature of the sensing device is dependent at least partly upon the amount of time.
The present invention also includes a method of efficiently preparing a heated electrode refrigerant detector, having a sensing device, for use, the method including the steps of: turning the detector on; increasing the actual temperature of the sensing device at a first rate of increase; monitoring at least one operating condition of the sensing device; and on the basis of an operating condition of the sensing device, increasing the actual temperature of the sensing device at a second rate of increase until a desired sensing device operating temperature is reached.
In features of this method, the sensing device is capable of generating a bias current, and the step of monitoring an operation condition includes monitoring the bias current; the step of increasing at a second rate occurs on the basis of the magnitude of the bias current being substantially equal to zero; the step of monitoring an operation condition includes monitoring absorbed moisture in the sensing device; the step of increasing at a second rate occurs on the basis of the substantially all of the initial quantity of absorbed moisture being evaporated; the step of monitoring absorbed moisture in the sensing device includes determining whether any absorbed moisture is present; the first rate of increase may be between 50 and 100 degrees Celsius per second; and the second rate of increase may be between 500 and 2000 degrees Celsius per second.
The present invention also includes a method of operating a heated electrode refrigerant detector, the method including the steps of: defining a sequence of desired temperature values; and adjusting the temperature of the detector according to the defined sequence.
In features of this method, the step of adjusting the temperature includes, for each desired temperature value in the sequence, the steps of determining the next desired temperature value in the sequence, controlling the temperature of the detector to effect the desired temperature value, monitoring the temperature of the detector to determine if the desired temperature value has been reached, and repeating the controlling and monitoring steps until the desired temperature value has been reached; the method includes the step of storing the desired temperature values in a memory; and the sequence of desired temperature values is selected to create a ramp function of temperature versus time.